For a majority of employers, the lack of ROI data has not stemmed incremental expansions in the types of services offered at the on-site centers.

While a majority of employers offering on-site health services to their employees still struggle to calculate their programs' worth, an even larger majority says senior managers support the programs, according to a Towers Watson & Co. study released July 31.

In a survey of 74 midsize to large employers sponsoring biometric screenings, urgent and primary care, physical therapy and other health services in on-site centers, 53 percent said they either don't know or haven't attempted to measure the actual return-on-investment of those services.

As nearly two-thirds of the companies surveyed said enhancing worker productivity was their primary motive for establishing an on-site health services center, 74 percent of the employers that have attempted to measure the ROI of those services use lost time on the job as their principal metric.

"Even though internal support is high, companies are finding it difficult to get their arms around how to measure the ROI of on-site health centers," Dr. Allan Khoury, a Cleveland-based senior consultant at Towers Watson, said in a statement accompanying the study.

Nevertheless, 75 percent of those same employers said senior leaders at their companies have supported the on-site services, while 58 percent said their employees are satisfied with the quality of the services offered.

"With employee health and productivity and cost control remaining paramount concerns, the future of on-site health centers remains very promising," Khoury said in the statement.

Since 2008, many employers have expanded access to their on-site health centers to include covered dependents while drawing down on services offered to retirees, contract and temporary employees. Thirty-six percent of employers in 2012 offer on-site health services to employees' spouses and children, compared with 25 percent in 2008.

"Treating covered dependents can provide as much value to an organization as treating an employee," said Patti Friedman, a senior consultant at Towers Watson's New York headquarters. "In fact, when evaluating the expected costs and savings of implementing an on-site health center, more use tends to translate into higher returns."

For a majority of employers, the lack of ROI data has not stemmed incremental expansions in the types of services offered at the on-site centers. Twenty-eight percent of employers said they plan to add telemedicine services in the next year, 9 percent said they intend to begin offering employee assistance programs, and 8 percent said they plan on offering urgent and primary care.

"It's important to note that on-site health centers will not be a fit for all companies," Greg Mansur, a Los Angeles-based senior consultant at Towers Watson, said in the statement. "However, many of those companies that have embraced them believe they can pay even greater dividends in the future."